My husband's neurologist told us that when he could no longer draw the clock with designated time (10 past 11, I think) it was time to stop driving. My husband gave it up easily. He was relieved.
I was determined for him to keep driving as long as he could, after the Alzheimer's Diagnosis. In retrospect, I think that was foolish. I had to watch him like a hawk. If he had had an accident, I doubt our insurance would have covered it.
Now he can't remember why he stopped driving and I tell him he failed the test. He accepts it and loves to be the passenger. He's a front seat driver now, always telling me what to do.